1. Field of the Invention
This invention relates to a paper sheet for ink jet printing which is adapted for use in ink jet printing systems using aqueous inks and which is unlikely to curl when printed and ensures high-quality print images thereon.
2. Description of the Related Art
For ink jet printing paper on which aqueous inks are applied, there are known ones which are suited for obtaining high-quality images and which have a coating layer by coating, onto a printing surface thereof, a coating composition comprising major proportion of a white pigment such as silica and an water-soluble resin. As is known, this type of printing paper can be divided into two classes including one wherein a large amount of a coating composition is applied onto the print side so that the ink is predominantly absorbed in the coating layer and the other class wherein a coating layer is formed in such a small amount that the ink is absorbed in both the coating layer and a paper substrate.
With the printing paper having a great amount of the coating, most of the ink is kept in the coating layer, so that a substantial dimensional change takes place between the coating layer and the substrate owing to the swelling through the water absorption of the water-soluble resin in the coating layer and owing to the shrinkage through the dehydration of the resin, thereby causing the paper to be considerably curled. On the other hand, with the printing paper whose coating layer is formed in a small amount, the ink infiltrates not only into the coating layer, but also into the inside of the substrate. This eventually leads to the dimensional change of the substrate through water absorption and dehydration of the fibers in the substrate, thereby causing the paper to be considerably curled or to become rugged.
Japanese Patent Laid-open No. Sho 62-202781 proposes an improvement of a printing paper having a great amount of the coating thereon wherein in order to suppress the curling, an ink-receiving layer is formed on a substrate having two or more layers and thus, the basis weight of the printing paper is made as great as 120 to 500 g/m.sup.2.
However, the technique set out in this gazette has the problem in practical application that the printing paper has such a great basis weight that it becomes too stiff or is in heavy body. This results in too high a buckling force of the paper at the time of its feed, thus presenting a problem on travellability such a miss feed especially for small-sized printers.
With respect to the printing paper having a coating layer in a small amount, the Japanese Patent Laid-open No. Hei 2-192984 proposes an improvement wherein a paper substrate is arranged as having a high surface smoothness, on which a coating layer is formed so that the resultant recording sheet has a water absorption within a given time greater than a predetermined level thereby avoiding rugged or curling defects on absorption of water.
However, in the improvement set forth in the Japanese Patent Laid-open No. Hei 2-192984, the water absorption of the substrate is set as being higher than a predetermined level, so that its ink absorptivity becomes high. This presents another problem that the ink readily arrives at the back side of the substrate, with a kind of setoff being undesirably conspicuous.